The present invention relates to conditioning of a polishing pad employed in chemical-mechanical polishing (sometimes referred to as "CMP"). More particularly, the present invention relates to conditioning sub-assemblies including conveying assemblies that employ either continuous feed or closed loop abrasive tape for effective conditioning of a polishing pad.
Chemical mechanical polishing (CMP) typically involves mounting a wafer faced down on a holder and rotating the wafer face against a polishing pad mounted on a platen, which in turn is rotating or is in an orbital state. A slurry containing a chemical that chemically interacts with the facing wafer layer and an abrasive that physically removes that layer is flowed between the wafer and the polishing pad or on the pad near the wafer. In semiconductor wafer fabrication, this technique is commonly applied to polish various wafer layers such as dielectric layers, metallization, etc.
Unfortunately after polishing on the same polishing pad for over a period of time, the polishing pad suffers from "pad glazing." As is well known in the art, pad glazing results when the particles eroded from the wafer surface along with the abrasives in the slurry tend to glaze or accumulate over the polishing pad. A glazed layer on the polished pad typically forms from eroded film and slurry particles that are embedded in the porosity or fibers of the polishing pad. Pad glazing is particularly pronounced during planarization of an oxide layer such as silicon dioxide layer (hereinafter referred to as "oxide CMP"). By way of example, during oxide CMP, eroded silicon dioxide particulate residue accumulates along with the abrasive particles from the slurry to form a glaze on the polishing pad. Pad glazing is undesirable because it reduces the polishing rate of the wafer surface and produces a non-uniformly polished wafer surface. The non-uniformity results because glazed layers are often unevenly distributed over a polishing pad surface.
One way of achieving and maintaining a high and stable polishing rate is by conditioning the polishing pad (hereinafter referred to as "pad conditioning") on a regular basis, i.e. either every time after a wafer has been polished or simultaneously during wafer CMP. FIG. 1 shows a side-sectional view of a cross-section of a polishing pad 102 undergoing conditioning according to current techniques of pad conditioning carried out in conventional or modern CMP systems. A surface of polishing pad 102 that is employed for oxide CMP has deposited on it a glazed oxide layer 104 resulting from pad glazing described above. During pad conditioning, a stream of water 106 is introduced over the polishing pad and a particle 108, for example, is dislodged from glazed layer 104 by mechanical action of a conditioning disk or arm (not shown to simplify illustration), described below in detail, to produce dislodged particles 108'. Furthermore, loose oxide particles 110 may also be found on the surface of polishing pad 104 as eroded oxide or slurry residue from CMP.
FIG. 2 shows some significant components of a conditioning sub-assembly 150 employed in conventional CMP systems. Conditioning sub-assembly 150 includes a conditioning arm 156 that is connected to a pneumatic cylinder (not shown to simplify illustration) on one end and on the other end has a connection 158, which connects to an end effector holder 160. An end effector 162 is secured at the top by end effector holder 160 and has mounted below it a conditioning disk 164 having abrasive particles 166.
During a typical conditioning cycle in the conventional CMP systems, a conditioning reagent is dispensed on polishing pad 152 through a separate outlet (not shown to simplify illustration) and conditioning disk 164 is lowered automatically to contact polishing pad 152, which may be rotating. The pneumatic cylinder then applies a downward force on conditioning disk 164 such that abrasive particles 166 engage polishing pad 152 as the conditioning disk along with end effector 162 moves on the polishing pad surface.
By way of example, in a conventional CMP system such as the Avanti 472, commercially available from Integrated Processing Equipment Corporation (IPEC) of Phoenix, Ariz., an oscillating motor (not shown to simplify illustration) coupled to connection 158 allows conditioning disk 164 to slide along a length of a stationary conditioning arm 156 in a radial direction from typically center to the edge of polishing pad 152. As another example, in the conventional CMP assembly of Strasbaugh 6DS-JP, commercially available from Strasbaugh of San Luis Obispo, Calif., conditioning disk 164 and end effector 162 are secured on conditioning arm 156, which moves in a radial direction of the polishing pad typically from center to the edge of polishing pad 152. The mechanical action of the conditioning disk in both examples attempts to break up and remove the glazed or accumulated particles coated on the polishing pad surface.
FIG. 3A shows some significant components of a conditioning sub-assembly 200, which is integrated into a modern CMP system, such as the AvantGaard 676, also commercially available from Integrated Processing Equipment Corporation (IPEC) of Phoenix, Ariz. Conditioning sub-assembly 200 includes a conditioning arm 204 that is disposed above a polishing pad 202 and capable of pivoting about a pivoting point 206. Conditioning arm 204, as shown in FIG. 3A, is typically longer in length than the diameter of polishing pad 202.
FIG. 3B shows a bottom view of conditioning arm 204 of FIG. 3A. The bottom surface of conditioning arm 204 includes a plurality of diamond abrasive particles 208, which are almost uniformly arranged on conditioning 204 arm such that if the conditioning arm contacts polishing pad 202, abrasive particles 208 engage a portion of the polishing pad. A manifold 210 having a plurality of openings 212 is mounted on both sides of conditioning arm 204, as shown in FIG. 3B. Openings 212 are designed to dispense a conditioning reagent on polishing pad 202 during pad conditioning and are therefore in communication with a reservoir of conditioning reagent (not shown to simplify illustration). In this configuration, openings 212 along with manifold 210 span the entire length of conditioning arm 204.
During a typical conditioning cycle in the modern CMP systems, a conditioning reagent is introduced on polishing pad 202 of FIG. 3A through openings 212 of FIG. 3B and conditioning arm 204 is lowered automatically to contact polishing pad 202, which may be in orbital motion. A pneumatic cylinder then applies a downward force on conditioning arm 204 such that abrasive particles 208 of FIG. 3B engage polishing pad 202 of FIG. 3A. Conditioning arm 204 typically sweeps back and forth across polishing pad 202 like a "windshield wiper blade" from one end (shown by conditioning arm 204') of the polishing pad to another (shown by conditioning arm 204") as shown in FIG. 3A to remove the glazed or accumulated particles coated on the polishing pad surface.
Unfortunately, pad conditioning carried out in conventional and modern CMP systems suffer from several drawbacks. By way of example, after repeated conditioning by the same conditioning or abrasive surface for a certain period of time in both the conventional and modern CMP systems, abrasive particles may dislodge from the conditioning disk surface or conditioning arm surface due to normal surface wear. The dislodged abrasive particles may remain on the polishing pad surface and scratch wafer surfaces that are subsequently polished on the polishing pad. This lowers the yield of the CMP process.
As another example, in the conventional CMP systems, in order to avoid scratching of the wafer surface, the conditioning disk may undergo a "wear-in" process before pad conditioning is carried out to make sure that all loosely secured abrasive particles are removed from the conditioning disk surface. This lowers the throughput of the wafer CMP process. Furthermore, when the abrasive particles are worn out, the conditioning disk must be replaced. In a typical semiconductor fabrication facility, for example, where several CMP systems are in operation, replacement costs for conditioning disks can be significant. Further still, determining whether a conditioning disk surface is worn out requires closely monitoring the changes on the wafer surfaces, which undergo polishing on the conditioned polishing pad.
As yet another example, in the modern CMP systems, typically the abrasive particles are secured on an adhesive tape that adheres to the conditioning arm. After the same abrasive tape is used for pad conditioning over a period of time, the abrasive tape often begins to peel off the conditioning arm and the pad conditioning process is not as effective, leading to poor and non-uniform film removal rates.
What is therefore needed is an improved pad conditioning sub-assembly that may be integrated into conventional and modern CMP systems.